This invention relates generally to electronic differential amplifiers and, more particularly, to a differential amplifier having output common mode offset voltage compensation.
Differential amplifiers typically have two inputs and are intended to amplify any difference of signal appearing between the two inputs. It is desirable that signals that are identical, with respect to both amplitude and time, and appear between the two terminals of the differential amplifier, not be amplified. However, it is not always possible to make corresponding parts of a differential amplifier electrically identical in all respects. Even if corresponding parts of a differential amplifier could be made to appear equal when first manufactured, such things as aging characteristics and/or temperature drift can cause the parts to exhibit different electrical parameters later on. This difference in electrical parameters can cause a common mode offset voltage. One solution to the problem, which has been proposed in the past, is to sample the differential output of the differential amplifier so that the bias currents can be adjusted accordingly to reduce the effects of the common mode offset. However, one of the shortcomings to such a solution is that when the bias currents are changed the gain of the differential amplifier also changes.
Accordingly, one of the objects of the present invention is to provide common mode offset compensation which does not affect the gain of the differential amplifier.
Another object of the present invention is to provide common mode offset compensation in a manner that permits controlling the bandwidth of the differential amplifier with a capacitor that is compatible with integrated circuit manufacturing processes.